1. Field of the Invention
The invention is directed to creation of a reverse look-up table from a forward look-up table for mapping between color spaces, in which the reverse look-up table includes input indexes that fall outside of a color gamut of a color device. In particular, the invention concerns creation of such a table wherein output color values for the input indexes that fall outside of the color gamut are extrapolated from color values that are interpolated from the forward look-up table near the gamut boundary.
2. Description of the Related Art
One goal of modern color management is generation of uniform output images using different color image output devices. In particular, it is desired that a color image generated for output using a first image output device would appear similar if output using a second different image output device.
For example, as shown in FIG. 1, computer system 1 could be used to generate a color image for output using color laser printer 2. Then, the image could be sent to computer system 4 for output using ink-jet printer 5, for example over network connection 7. It is preferable that the images output by color laser printer 2 and ink-jet printer 5 both appear similar.
One conventional technique for attempting to ensure similar output images from different image output devices is illustrated in FIG. 2. In that figure, device dependent image 9 is defined in a device dependent color space corresponding to a first color device. Examples of the device dependent color space include CMY (cyan, magenta and yellow), CMYK (cyan, magenta, yellow and black) and RGB (red, green and blue) color spaces. Examples of the first color device include color laser printer 2 or a color monitor for computer system 1 shown in FIG. 1.
The device dependent image is transferred across connection 10 for output by a second color device. For example, device independent image 9 could be transferred across network connection 7 for output by ink-jet printer 5 or a color monitor for computer system 4 shown in FIG. 1.
In order to transform device dependent image 9 for output by the second color device, the device dependent image is first input to look-up table A (LUTA) 11. LUTA 11 transforms device dependent image 9 into device independent image 13 in a device independent appearance color space. The image is transformed in accordance with color characteristics of the first color device for which LUTA 11 is defined. Examples of device independent appearance color spaces are Lab, LCH and JCh color spaces. (The JCh color space preferably is defined in accordance with the CIECAM 97S appearance model).
Then, device independent image 13 is transformed by reverse look-up table B (RLUTB) 15 into device dependent image 17 in a device dependent color space. As indicated by the dashed arrow in FIG. 2, RLUTB is derived from look-up table B (LUTB) 16, which embodies the color characteristics of the second color device.
FIG. 3 illustrates LUTA 11 and RLUTB 15 in more detail. For the sake of simplicity, color values are represented in one dimension in FIG. 3.
LUTA 11 has input indexes for device dependent color stimulus values 21 and outputs for device independent color response values 22. The color stimulus values span all possible color stimulus values (here 0 to 9), and the color response values span all corresponding color response values in color gamut A 23 for the first color device.
Likewise, RLUTB 15 has input indexes for device independent color response values 25 and outputs for device dependent color stimulus values 26. The indexes for RLUTB 15 include color response values outside of color gamut B 27 for the second color device. RLUTB 15 therefore can be used with images whose colors extend beyond the colors that can be output by the second device. In order to accommodate those colors, for example a color response value of 1.6 at a lower end of color gamut A 23, RLUTB 15 incorporates gamut mapping 31 for out-of-gamut color response values.
The foregoing arrangement suffers from at least three deficiencies. First, gamut mapping 31 distorts the ends of RLUTB 15, preventing a mathematically accurate mapping from device independent color response values 25 to device dependent color stimulus values 26.
Second, in order to change the gamut mapping, it is necessary to rederive RLUTB 15 from LUTB 16. Rederiving RLUTB 15 can be computationally expensive.
Third, all possible input color response values for all possible color devices must be accounted for in the gamut mapping. In practice, however, many devices are incapable of creating many of these theoretically possible color response avalues. As a result, conventional gamut mapping which is incorporated into the reverse look-up table tends to overcompress the color gamut.
The foregoing deficiencies also arise in other contexts wherein a reverse look-up table is derived from a forward look-up table. For example, when an image generated for output using a monitor is instead output using a printer, a reverse look-up table for the printer can be used in the color mapping for the image. If the reverse look-up table is derived from a forward look-up table, then the deficiencies described above may be encountered.